The present invention relates to a spindle motor used as an apparatus for rotatively driving a hard disk or the like.
A spindle motor disclosed in, for example, Japanese Patent Publication No. 8-4769A is known as a spindle motor used as apparatus for rotatively driving a recording medium such as a hard disk. As shown in FIG. 7, this spindle motor is mainly comprised of a stator assembly 100 and a rotor assembly 120 having driving magnets 125. The rotor assembly 120 has a hub 122 secured to an upper end portion of a rotary shaft 121 by means of press-fitting, shrinkage fitting, or the like. Meanwhile, the stator assembly 100 has stator cores 116 each formed by winding a coil 117 around a respective salient pole portion. These stator cores 116 are fitted to an outer peripheral portion of a bearing holder 115.
A bearing sleeve 113 is fitted in an inner peripheral portion of the bearing holder 115. Radial bearing portions RBa and RBb serving as bearing surfaces for generating hydrodynamic pressure are formed on an inner peripheral surface of the bearing sleeve 113 in such a manner as to be spaced apart from each other in the axial direction. A lubricating fluid 105 such as oil undergoes a pressure rise due to the pumping action of dynamic pressure generating grooves (not shown) when the rotary shaft 121 rotates, and the rotary shaft 121 and the hub 122 are pivotally supported by the hydrodynamic pressure generated by the lubricating fluid 105.
Further, a thrust plate 126 constituting a thrust hydrodynamic bearing portion is press-fitted and secured to the rotary shaft 121. Further, a counter plate 114 is fixed at an open end of the bearing holder 115 of a frame 111 through a mechanical coupling means such as fixing screws 106. The thrust plate 126 is placed between a lower end face of the bearing sleeve 113 and an inner bottom surface of the counter plate 114, and as the lubricating fluid 105 is present in this space, the rotary shaft 121 is stably supported in the thrust direction by the hydrodynamic pressure generated by the lubricating fluid 105.
In recent years, a trend toward compact and thin spindle motors for rotatively driving recording medium disks are rapidly underway. In conjunction with this trend, the bearing member (bearing sleeve 113) supporting the shaft 121 is formed of a metallic material different from the metallic material composing the fixing frame 111. One reason for this is that a metal excelling in workability is adopted as the metallic material composing the bearing sleeve 113 so that the inside-diameter portion of the bearing sleeve 113 can be machined satisfactorily. In this case, the bearing sleeve 113 formed of a different type of metallic material is integrally joined to the fixing frame 111 by means of press-fitting, shrinkage fitting or the like.
In a spindle motor in which different types of metallic material are integrally joined together, if an electrolyte having a large dielectric constant, such as water, penetrates the joint, a local battery is formed between these metallic materials of different types, and anodic dissolution occurs due to the local battery, resulting in the so-called potential difference corrosion. The portion where such potential difference corrosion occurs is scattered in due course of time in the form of dust, and causes damage to the recording medium disk or the magnetic head. Accordingly, in the case of an apparatus for which cleanliness is required, such as a hard disk drive (HDD), it is desirable to reliably prevent the occurrence of the aforementioned potential difference corrosion.
In recent years when motors are required to be thinner, it has become impossible to secure a sufficient joining length in the joining of the rotary shaft and the thrust plate and in the joining of the rotary shaft and the hub. Consequently, there have arisen problems in that it is difficult to obtain desired shock-resisting performance (e.g., 1,000 G or more) and joining strength capable of withstanding an external stress during assembly, thereby making it difficult to produce a thin motor.
For instance, in FIG. 7, various joining methods are adopted in joining the counter plate 114 and the frame 111 or in joining the counter plate 114 and the bearing sleeve 113. In a case where the fixing screws 106 shown in FIG. 7 are used to effect fastening, the heads of the fixing screws 106 hinder the attempt to produce a thin motor. In a case where the counter plate 114 is fixed by a calking method, the calked portion must be made to project from the bottom surface of the counter plate 114, which also hinders the attempt to produce a thin motor. Further, in a case where the counter plate 114 is fixed by a press-fitting method, since a sufficient joining length cannot be obtained, the joining strength lacks.
A primary object of the invention is to provide a spindle motor which makes it possible to prevent by a simple arrangement the potential difference corrosion between a bearing member and another member which are formed of metallic materials of different types.
A secondary object of the invention is to provide a spindle motor which can be made thin by increasing the joining strength even in the case of a part whose joining length is short.
In accordance with the invention, the arrangement is provided such that a potential-difference alleviating member for alleviating and lowering the potential difference, which is an energy difference between a rotating or fixed bearing member and a rotary hub or a fixing frame which are formed of metals of different types, is interposed between the two members so as to prevent the occurrence or advance of potential difference corrosion. Accordingly, the working environment of an apparatus such as a hard disk drive (HDD), in particular, for which cleanliness is required, can be made favorable, and the reliability of the apparatus can be improved.
Further, in accordance with the invention, the arrangement is provided such that an insulating resin coating film or a passivation film is interposed between a rotating or fixed bearing member and a rotary hub or a fixing frame which are formed of metals of different types, so as to prevent the occurrence of a local battery and prevent the occurrence or advance of potential difference corrosion. Accordingly, the working environment of an apparatus such as a hard disk drive (HDD), in particular, for which cleanliness is required, can be made favorable, and the reliability of the apparatus can be improved.
Furthermore, in accordance with the invention, the arrangement is provided such that relief portions are respectively provided at a joining interface between the rotary shaft and the thrust plate and a joining interface between the bearing member and the counter plate or a joining interface the fixing frame and the counter plate, and the respective members are welded in the relief portions so as to be integrated. Accordingly, even if the joining length of the members is relatively short, it is possible to obtain a sufficient joining strength and improve the shock resistance of the motor itself. As a result, the perpendicularity of the thrust plate with respect to the rotary shaft, for example, can be maintained stably, and the reliability of the motor can be improved.